The cell searching has to be performed whenever all user terminals in the mobile communication system are powered on or the cell is re-selected. Accordingly, a method for cell searching is provided in the standarded of Third Generation Mobile Communication System.
Take TD-SCDMA system as an example in TDD (Time Division Duplex) system to describe its frame structure before introducing the cell searching method. FIG. 1 is a schematic diagram illustrating the frame structure in TD-SCDMA system and the cell searching therein. A TD-SCDMA signal is composed of periodic time units on the basis of time. A basic time unit is regarded as a “wireless frame” (a frame) of 10 ms in length; each “wireless frame” can be divided into two subframes of 5 ms in length; each subframe is further divided into seven regular time slots TS0-TS6 and three special time slots located between TS0 and TS1, wherein, the traffic is transferred in the regular time slots and three special time slots include DwPTS (Downlink Pilot Time Slot), UpPTS (uplink Pilot Time Slot) and the Guard band (G). According to TD-SCDMA standarded, TS0 is usually designated as the Downlink direction, and TS1 is usually designated as the Uplink direction.
The cell searching procedure comprises steps as follows: firstly, the data of one subframe is received at the working carrier frequency and performing AGC (Automatic Gain Control) is immediately followed, so that the maximum level of the signal received within the time of the subframe (T=0 ms to T=5 ms) will not cause saturation of the receiver (located between the minimum receiving level and the saturation level of the receiver as shown in the figure); secondly, the DwPTS position is acquired by detecting a sliding time window point-by-point, e.g., the arrival time to as shown in the figure, based on the structure of the TD-SCDMA frame having no transmission power in the guard time before and after DwPTS, and the downlink synchronization and carrier tracking according to the timing of said DwPTS are acquired; finally, BCCH (Broadcast Control Channel) information is received and read in TS0 time slot until the cell searching is completed.
The time window decision method can be adopted to acquire the DwPTS position by detecting a sliding time window point-by-point. The time window decision method includes: to determine the length of the sliding time window as eight symbols based on the feature that the DwPTS code length is fixed as four symbols, and to judge the DwPTS position according to the criterion that the levels of the four middle symbols are higher than that of the two symbols at two sides thereof.
In practical user terminals, the dynamic range of the receiver is limited, therefore, the receiver usually uses the automatic gain control technology. As the cell searching begins, the receiver performs the automatic gain control immediately after receiving a subframe data to ensure that the maximum level of signal (including interference) in the received subframe will not cause saturation of the receiver. In FIG. 1, the vertical line areas represent the level of the received downlink signal from the base station, and the horizontal line areas represent the level of the received uplink signal of other working user terminals.
FDD (Frequency Division Duplexing) system adopts different working carrier frequencies in receiving and transmitting, and TDD (Time Division Duplexing) system only uses one carrier frequency, i.e., a same carrier frequency is adopted in receiving and transmitting. The normal AGC manner is adopted to ensure that the level of the signal in the received subframe at the terminal does not cause saturation of the receiver as the automatic gain control is performed immediately, which, however, will not only make the cell initial searching difficult but also leadfailure of it when the following situation: the user terminal performing the cell searching will receive not only the downlink signal from the base station but also the uplink signal of other working user terminals, what is more, if the user terminal locates around the border of the cell which adopts a plurality of same carrier frequencies for working, it will also receive the downlink signal of the base station in neighbouring cells as well as the uplink signal of the working user terminals in said neighbouring cells; if one or more user terminals in the vicinity of the user terminal performing the cell searching are calling, and their working frequencies are same, the uplink signal from the user terminal may be far higher than the downlink signal from the base station. The presence of the interference makes the cell searching more difficult in TDD system.
To solve the above problems, the common way is adopted to increase the receiving dynamic range of the receiver nowadays, and ADC (Anolog-to-Digital Converter) with more bits must be used accordingly, which leads a high cost of the device. Moreover, the working voltage of the user terminal is very low, the ADC bits being used are difficult to exceed ten bits, and the dynamic range is difficult to exceed 60 dB. Therefore, it is impossible to realize the object of increasing the desired dynamic range of the receiver.
In a practical mobile communication network, the AGC range of the receiver at TD-SCDMA user terminal can exceed 80 dB, but it is only 60 dB at a certain gain (assuming using ADC with 10 bit accuracy). Several cases are hereinafter described in conjunction with FIG. 2 to illustrate the problems that occur in the initial cell searching when adopting said normal automatic gain control method.
FIG. 2A shows a typical case. In a subframe (5 ms), the user obtains the downlink signal mainly from the base station which is indicated by the vertical line areas, and the signal of other working users is very low. A good receiving effect can be obtained and the initial cell searching can be realized easily once the aforementioned normal automatic gain control technology is used. The block area indicates the thermal noise level as shown in FIG. 2.
FIG. 2B shows a case that a user is located far from the base station and another working user is nearby. When another working user terminal (indicated by the horizontal line area in the figure) exists in the vicinity of a user (indicated by the vertical line area), and the signal level of said another working user terminal may reach −30 dB or more, the gain control is adjusted immediately to be capable of receiving the signal level of the subframe (5 ms) normally not to lead saturation of the receiver if the normal AGC technology is employed. Meanwhile, the signal (DwPTS in the figure) is lower than the noise (ADC quantification noise level indicated by the shadow in the figure) due to the limitation of the ADC sampling accuracy, which leads failure of the initial cell searching.
FIG. 2C shows a case that a user locates at the edge of the cell and there are interferences from neighbouring cells. although there is no interference of neighbouring users, the interference from neighbouring cells is severe when the user locates at the edge of a cell, and SNR is very low. The base stations in neighbouring cells are synchronized, so the present positions of DwPTS of the subframes in respective cells are adjacent to each other, e.g., DwPTS in different cells as shown in the figure. However, the increase of the width of DwPTS received by the user is not large, and the feature that both guards of DwPTS are still exists. Therefore, the cell searching is difficult to be carried our, but can be realized when adopting the normal AGC technology.
The last possible case is the combination by those shown in FIG. 2B and FIG. 2C. For the same reason as shown in FIG. 2B the initial cell searching will definitely be a failure.
In conclusion, when the initial cell searching is performed, the user terminal performs the automatic gain control immediately when receiving a subframe signal, therefore, it is difficult to realize the initial cell searching in the case of other working user terminals being in the vicinity of said user terminal, which may even lead failure of the cell searching.